Multilayer polymer film with additional coatings or layers

ABSTRACT

A multilayer polymer film has an optical stack including a plurality of alternating polymer layers with skin layers having mechanical, optical, or chemical properties differing from those of the layers in the optical stack, wherein the refractive indices in the in-plane direction n x  and n y , and the refractive index in the thickness direction n z  for each layer are all selected to obtain optical effects such as reflection, transmission, and/or polarization.

RELATED APPLICATION

This a continuation of U.S. patent application Ser. No. 08/494,416 filedJun. 26, 1995 abandoned.

BACKGROUND OF THE INVENTION

Multilayer optical stacks are well-known for providing a wide variety ofoptical properties. Such multilayer stacks may act as reflectivepolarizers or mirrors, reflecting light of all polarizations. They mayalso function as wavelength selective reflectors such as “cold mirrors”that reflect visible light but transmit infrared or “hot mirrors” thattransmit visible and reflect infrared. Examples of a wide variety ofmultilayer stacks that may be constructed are included in U.S. patentapplication Ser. No. 08/402,041 filed Mar. 10, 1995 now U.S. Pat. No.5,882,774.

A problem with multilayer stacks as known in the art is that the stacksthemselves may not have all of the physical, chemical, or opticalproperties desired. Some way of otherwise supplying these desirableproperties would therefore be useful.

SUMMARY OF THE INVENTION

According to one embodiment of the invention a multilayer film hasadhered to one or both of its major surfaces at least one additionallayer selected for mechanical, chemical, or optical properties thatdiffer from the properties of the materials of the layers of the opticalstack.

According to another embodiment of the invention a multilayer film hasadhered to one or both of its surfaces an additional layer that willprotect the multilayer optical stack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 2 show the preferred multilayer optical film;

FIGS. 3 through 8 show transmission spectra for the multilayer opticalfilms of Examples 1 through 6;

FIG. 9 shows a multilayer film of the invention having an additionallayer adhered to one of its major surfaces;

FIG. 10 shows a multilayer film according to the invention havingadditional layers adhered to both of its major surfaces; and

FIG. 11 shows a multilayer film having one additional layer adhered toone of its major surfaces and two additional layers adhered to its othermajor surface.

DETAILED DESCRIPTION

Multilayer Optical Film

The advantages, characteristics and manufacturing of multilayer opticalfilms are most completely described in the above-mentioned copending andcommonly-assigned U.S. patent application Ser. No. 08/402,041, filedMar. 10, 1995, titled OPTICAL FILM, which is incorporated herein byreference. The multilayer optical film is useful, for example, as highlyefficient mirrors and/or polarizers. A relatively brief description ofthe properties and characteristics of the multilayer optical film ispresented below followed by a description of illustrative embodiments ofbacklight systems using the multilayer optical film according to thepresent invention.

Multilayer optical films as used in conjunction with the presentinvention exhibit relatively low absorption of incident light, as wellas high reflectivity for off-axis as well as normal light rays. Theseproperties generally hold whether the films are used for pure reflectionor reflective polarization of light. The unique properties andadvantages of the multilayer optical film provides an opportunity todesign highly-efficient backlight systems which exhibit low absorptionlosses when compared to known backlight systems.

An exemplary multilayer optical film of the present invention asillustrated in FIGS. 1A and 1B includes a multilayer stack 10 havingalternating layers of at least two materials 12 and 14. At least one ofthe materials has the property of stress induced birefringence, suchthat the index of refraction (n) of the material is affected by thestretching process. FIG. 1A shows an exemplary multilayer stack beforethe stretching process in which both materials have the same index ofrefraction. Light ray 13 experiences relatively little change in indexof refraction and passes through the stack. In FIG. 1B, the same stackhas been stretched, thus increasing the index of refraction of material12. The difference in refractive index at each boundary between layerswill cause part of ray 15 to be reflected. By stretching the multilayerstack over a range of uniaxial to biaxial orientation, a film is createdwith a range of reflectivities for differently oriented plane-polarizedincident light. The multilayer stack can thus be made useful asreflective polarizers or mirrors.

Multilayer optical films constructed according to the present inventionexhibit a Brewster angle (the angle at which reflectance goes to zerofor light incident at any of the layer interfaces) which is very largeor is nonexistent for the polymer layer interfaces. In contrast, knownmultilayer polymer films exhibit relatively small Brewster angles atlayer interfaces, resulting in transmission of light and/or undesirableiridescence. The multilayer optical films according to the presentinvention, however, allow for the construction of mirrors and polarizerswhose reflectivity for p polarized light decrease slowly with angle ofincidence, are independent of angle of incidence, or increase with angleof incidence away from the normal. As a result, multilayer stacks havinghigh reflectivity for both s and p polarized light over a widebandwidth, and over a wide range of angles can be achieved.

FIG. 2 shows two layers of a multilayer stack, and indicates the threedimensional indices of refraction for each layer. The indices ofrefraction for each layer are n1x, n1y, and n1z for layer 102, and n2x,n2y, and n2z for layer 104. The relationships between the indices ofrefraction in each film layer to each other and to those of the otherlayers in the film stack determine the reflectance behavior of themultilayer stack at any angle of incidence, from any azimuthaldirection. The principles and design considerations described in U.S.patent application Ser. No. 08/402,041 can be applied to createmultilayer stacks having the desired optical effects for a wide varietyof circumstances and applications. The indices of refraction of thelayers in the multilayer stack can be manipulated and tailored toproduce the desired optical properties.

Referring again to FIG. 1B, the multilayer stack 10 can include tens,hundreds or thousands of layers, and each layer can be made from any ofa number of different materials. The characteristics which determine thechoice of materials for a particular stack depend upon the desiredoptical performance of the stack. The stack can contain as manymaterials as there are layers in the stack. For ease of manufacture,preferred optical thin film stacks contain only a few differentmaterials.

The boundaries between the materials, or chemically identical materialswith different physical properties, can be abrupt or gradual. Except forsome simple cases with analytical solutions, analysis of the latter typeof stratified media with continuously varying index is usually treatedas a much larger number of thinner uniform layers having abruptboundaries but with only a small change in properties between adjacentlayers.

The preferred multilayer stack is comprised of low/high index pairs offilm layers, wherein each low/high index pair of layers has a combinedoptical thickness of ½ the center wavelength of the band it is designedto reflect. Stacks of such films are commonly referred to as quarterwavestacks. For multilayer optical films concerned with the visible and thenear infrared wavelengths, a quarterwave stack design results in each ofthe layers in the multilayer stack having an average thickness of notmore than 0.5 microns.

In those applications where reflective films (e.g. mirrors) are desired,the desired average transmission for light of each polarization andplane of incidence generally depends upon the intended use of thereflective film. One way to produce a multilayer mirror film is tobiaxially stretch a multilayer stack. For a high efficiency reflectivefilm, average transmission along each stretch direction at normalincidence over the visible spectrum (380-750 nm) is desirably less than10 percent (reflectance greater than 90 percent), preferably less than 5percent (reflectance greater than 95 percent), more preferably less than2 percent (reflectance greater than 98 percent), and even morepreferably less than 1 percent (reflectance greater than 99 percent).The average transmission at 60 degrees from the normal from 380-750 nmis desirably less than 20 percent (reflectance greater than 80 percent),preferably less than 10 percent (reflectance greater than 90 percent),more preferably less than 5 percent (reflectance greater than 95percent), and even more preferably less than 2 percent (reflectancegreater than 98 percent), and even more preferably less than 1 percent(reflectance greater than 99 percent).

In addition, asymmetric reflective films may be desirable for certainapplications. In that case, average transmission along one stretchdirection may be desirably less than, for example, 50 percent, while theaverage transmission along the other stretch direction may be desirablyless than, for example 20 percent, over a bandwidth of, for example, thevisible spectrum (380-750 nm), or over the visible spectrum and into thenear infrared (e.g., 380-850 nm).

Multilayer optical films can also be designed to operate as reflectivepolarizers. One way to produce a multilayer reflective polarizer is touniaxially stretch a multilayer stack. The resulting reflectivepolarizers have high reflectivity for light with its plane ofpolarization parallel to one axis (in the stretch direction) for a broadrange of angles of incidence, and simultaneously have low reflectivityand high transmissivity for light with its plane of polarizationparallel to the other axis (in the non-stretch direction) for a broadrange of angles of incidence. By controlling the three indices ofrefraction of each film, nx, ny and nz, the desired polarizer behaviorcan be obtained.

For many applications, the ideal reflecting polarizer has highreflectance along one axis (the so-called extinction axis) and zeroreflectance along the other (the so-called transmission axis), at allangles of incidence. For the transmission axis of a polarizer, itgenerally desirable to maximize transmission of light polarized in thedirection of the transmission axis over the bandwidth of interest andalso over the range of angles of interest.

The average transmission at normal incidence for a polarizer in thetransmission axis across the visible spectrum (380-750 nm for abandwidth of 300 nm) is desirably at least 50 percent, preferably atleast 70 percent, more preferably at least 80 percent, and even morepreferably at least 90 percent. The average transmission at 60 degreesfrom the normal (measured along the transmission axis for p-polarizedlight) for a polarizer from 380-750 nm is desirably at least 50 percent,preferably at least 70 percent, more preferably at least 80 percent, andeven more preferably at least 90 percent.

The average transmission for a multilayer reflective polarizer at normalincidence for light polarized in the direction of the extinction axisacross the visible spectrum (380-750 nm for a bandwidth of 300 nm) isdesirably at less than 50 percent, preferably less than 30 percent, morepreferably less than 15 percent, and even more preferably less than 5percent. The average transmission at 60 degrees from the normal(measured along the transmission axis for p-polarized light) for apolarizer for light polarized in the direction of the extinction axisfrom 380-750 nm is desirably less than 50 percent, preferably less than30 percent, more preferably less than 15 percent, and even morepreferably less than 5 percent.

For certain applications, high reflectivity for p-polarized light withits plane of polarization parallel to the transmission axis atoff-normal angles are preferred. The average reflectivity for lightpolarized along the transmission axis should be more than 20 percent atan angle of at least 20 degrees from the normal.

In addition, although reflective polarizing films and asymmetricreflective films are discussed separately herein, it should beunderstood that two or more of such films could be provided to reflectsubstantially all light incident on them (provided they are properlyoriented with respect to each other to do so). This construction istypically desired when the multilayer optical film is used as areflector in a backlight system according to the present invention.

If some reflectivity occurs along the transmission axis, the efficiencyof the polarizer at off-normal angles may be reduced. If thereflectivity along the transmission axis is different for variouswavelengths, color may be introduced into the transmitted light. One wayto measure the color is to determine the root mean square (RMS) value ofthe transmissivity at a selected angle or angles over the wavelengthrange of interest. The percent RMS color, C_(RMS), can be determinedaccording to the equation:$C_{RMS} = \frac{\int_{\lambda 1}^{\lambda 2}{\left( \left( {T - \overset{\_}{T}} \right)^{2} \right)^{1/2}\quad {\lambda}}}{\overset{\_}{T}}$

where the range 11 to 12 is the wavelength range, or bandwidth, ofinterest, T is the transmissivity along the transmission axis, and{overscore (T)} is the average transmissivity along the transmissionaxis in the wavelength range of interest. For applications where a lowcolor polarizer is desirable, the percent RMS color should be less than10 percent, preferably less than 8 percent, more preferably less than3.5 percent, and even more preferably less than 2 percent at an angle ofat least 30 degrees from the normal, preferably at least 45 degrees fromthe normal, and even more preferably at least 60 degrees from thenormal.

Preferably, a reflective polarizer combines the desired percent RMScolor along the transmission axis for the particular application withthe desired amount of reflectivity along the extinction axis across thebandwidth of interest. For polarizers having a bandwidth in the visiblerange (400-700 nm, or a bandwidth of 300 nm), average transmission alongthe extinction axis at normal incidence is desirably less than 40percent, more desirably less than 25 percent, preferably less than 15percent, more preferably less than 5 percent and even more preferablyless than 3 percent.

Materials Selection and Processing

With the design considerations described in the above mentioned U.S.patent application Ser. No. 08/402,041, one of ordinary skill willreadily appreciate that a wide variety of materials can be used to formmultilayer reflective films or polarizers according to the inventionwhen processed under conditions selected to yield the desired refractiveindex relationships. The desired refractive index relationships can beachieved in a variety of ways, including stretching during or after filmformation (e.g., in the case of organic polymers), extruding (e.g., inthe case of liquid crystalline materials), or coating. In addition, itis preferred that the two materials have similar rheological properties(e.g., melt viscosities) such that they can be co-extruded.

In general, appropriate combinations may be achieved by selecting, asthe first material, a crystalline or semi-crystalline, or liquidcrystalline material, preferably a polymer. The second material, inturn, may be crystalline, semi-crystalline, or amorphous. The secondmaterial may have a birefringence opposite to or the same as that of thefirst material. Or, the second material may have no birefringence. Itshould be understood that in the polymer art it is generally recognizedthat polymers are typically not entirely crystalline, and therefore inthe context of the present invention, crystalline or semi-crystallinepolymers refer to those polymers that are not amorphous and includes anyof those materials commonly referred to as crystalline, partiallycrystalline, semi-crystalline, etc. The second material may have abirefringence opposite to or the same as that of the first material. Or,the second material may have no birefringence.

Specific examples of suitable materials include polyethylene naphthalate(PEN) and isomers thereof (e.g., 2,6-, 1,4-, 1,5-, 2,7-, and 2,3-PEN),polyalkylene terephthalates (e.g., polyethylene terephthalate,polybutylene terephthalate, and poly-1,4-cyclohexanedimethyleneterephthalate), polyimides (e.g., polyacrylic imides), polyetherimides,atactic polystyrene, polycarbonates, polymethacrylates (e.g.,polyisobutyl methacrylate, polypropylmethacrylate,polyethylmethacrylate, and polymethylmethacrylate), polyacrylates (e.g.,polybutylacrylate and polymethylacrylate), syndiotactic polystyrene(sPS), syndiotactic poly-alpha-methyl styrene, syndiotacticpolydichlorostyrene, copolymers and blends of any of these polystyrenes,cellulose derivatives (e.g., ethyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, and cellulosenitrate), polyalkylene polymers (e.g., polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinatedpolymers (e.g., perfluoroalkoxy resins, polytetrafluoroethylene,fluorinated ethylene-propylene copolymers, polyvinylidene fluoride, andpolychlorotrifluoroethylene), chlorinated polymers (e.g., polyvinylidenechloride and polyvinylchloride), polysulfones, polyethersulfones,polyacrylonitrile, polyamides, silicone resins, epoxy resins,polyvinylacetate, polyether-amides, ionomeric resins, elastomers (e.g.,polybutadiene, polyisoprene, and neoprene), and polyurethanes. Alsosuitable are copolymers, e.g., copolymers of PEN (e.g., copolymers of2,6-, 1,4-, 1,5-, 2,7-, and/or 2,3-naphthalene dicarboxylic acid, oresters thereof, with (a) terephthalic acid, or esters thereof; (b)isophthalic acid, or esters thereof; (c) phthalic acid, or estersthereof; (d) alkane glycols; (e) cycloalkane glycols (e.g., cyclohexanedimethane diol); (f) alkane dicarboxylic acids; and/or (g) cycloalkanedicarboxylic acids (e.g., cyclohexane dicarboxylic acid)), copolymers ofpolyalkylene terephthalates (e.g., copolymers of terephthalic acid, oresters thereof, with (a) naphthalene dicarboxylic acid, or estersthereof; (b) isophthalic acid, or esters thereof; (c) phthalic acid, oresters thereof; (d) alkane glycols; (e) cycloalkane glycols (e.g.,cyclohexane dimethanel diol); (f) alkane dicarboxylic acids; and/or (g)cycloalkane dicarboxylic acids (e.g., cyclohexane dicarboxylic acid)),and styrene copolymers (e.g., styrene-butadiene copolymers andstyrene-acrylonitrile copolymers), 4,4′-bibenzoic acid and ethyleneglycol. In addition, each individual layer may include blends of two ormore of the above-described polymers or copolymers (e.g., blends of sPSand atactic polystyrene). The coPEN described may also be a blend ofpellets where at least one component is a polymer based on naphthalenedicarboxylic acid and other components are other polyesters orpolycarbonates, such as a PET, a PEN or a coPEN.

Particularly preferred combinations of layers in the case of polarizersinclude PEN/coPEN, polyethylene terephthalate (PET)/coPEN, PEN/sPS,PET/sPS, PEN/Estar, and PET/Estar, where “coPEN” refers to a copolymeror blend based upon naphthalene dicarboxylic acid (as described above)and Estar is polycyclohexanedimethylene terephthalate commerciallyavailable from Eastman Chemical Co.

Particularly preferred combinations of layers in the case of reflectivefilms include PET/Ecdel, PEN/Ecdel, PEN/sPS, PEN/THV, PEN/co-PET, andPET/sPS, where “co-PET” refers to a copolymer or blend based uponterephthalic acid (as described above), Ecdel is a thermoplasticpolyester commercially available from Eastman Chemical Co., and THV is afluoropolymer commercially available from Minnesota Mining andManufacturing Company, St. Paul, Minn.

The number of layers in the film is selected to achieve the desiredoptical properties using the minimum number of layers for reasons offilm thickness, flexibility and economy. In the case of both polarizersand reflective films, the number of layers is preferably less than10,000, more preferably less than 5,000, and even more preferably lessthan 2,000.

As discussed above, the ability to achieve the desired relationshipsamong the various indices of refraction (and thus the optical propertiesof the multilayer film) is influenced by the processing conditions usedto prepare the multilayer film. In the case of organic polymers whichcan be oriented by stretching, the films are generally prepared byco-extruding the individual polymers to form a multilayer film and thenorienting the film by stretching at a selected temperature, optionallyfollowed by heat-setting at a selected temperature. Alternatively, theextrusion and orientation steps may be performed simultaneously. In thecase of polarizers, the film is stretched substantially in one direction(uniaxial orientation), while in the case of reflective films the filmis stretched substantially in two directions (biaxial orientation).

The film may be allowed to dimensionally relax in the cross-stretchdirection from the natural reduction in cross-stretch (equal to thesquare root of the stretch ratio); it may simply be constrained to limitany substantial change in cross- stretch dimension; or it may beactively stretched in the cross-stretch dimension. The film may bestretched in the machine direction, as with a length orienter, or inwidth using a tenter.

The pre-stretch temperature, stretch temperature, stretch rate, stretchratio, heat set temperature, heat set time, heat set relaxation, andcross-stretch relaxation are selected to yield a multilayer film havingthe desired refractive index relationship. These variables areinterdependent; thus, for example, a relatively low stretch rate couldbe used if coupled with, e.g., a relatively low stretch temperature. Itwill be apparent to one of ordinary skill how to select the appropriatecombination of these variables to achieve the desired multilayer film.In general, however, a stretch ratio in the range from 1:2 to 1:10 (morepreferably 1:3 to 1:7) in the stretch direction and from 1:0.2 to 1:10(more preferably from 1:0.2 to 1:7) orthogonal to the stretch directionis preferred.

Suitable multilayer films may also be prepared using techniques such asspin coating (e.g., as described in Boese et al., J. Polym. Sci.: PartB, 30:1321 (1992) for birefringent polyimides, and vacuum deposition(e.g., as described by Zang et. al., Appl. Phys. Letters, 59:823 (1991)for crystalline organic compounds; the latter technique is particularlyuseful for certain combinations of crystalline organic compounds andinorganic materials.

Exemplary multilayer reflective mirror films and multilayer reflectivepolarizers will now be described in the following examples.

EXAMPLE 1 (PEN:THV 500, 449, Mirror)

A coextruded film containing 449 layers was made by extruding the castweb in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. A Polyethylene naphthalate (PEN) with anIntrinsic Viscosity of 0.53 dl/g (60 weight percent phenol/40 weightpercent dichlorobenzene) was delivered by one extruder at a rate of 56pounds per hour and THV 500 (a fluoropolymer available from MinnesotaMining and Manufacturing Company) was delivered by another extruder at arate of 11 pounds per hour. The PEN was on the skin layers and 50percent of the PEN was present in the two skin layers. The feedblockmethod was used to generate 57 layers which was passed through threemultipliers producing an extrudate of 449 layers. The cast web was 20mils thick and 12 inches wide. The web was later biaxially orientedusing a laboratory stretching device that uses a pantograph to grip asquare section of film and simultaneously stretch it in both directionsat a uniform rate. A 7.46 cm square of web was loaded into the stretcherat about 100 degrees C and heated to 140 degrees C in 60 seconds.Stretching then commenced at 10 percent/sec (based on originaldimensions) until the sample was stretched to about 3.5×3.5. Immediatelyafter the stretching the sample was cooled by blowing room temperatureair at it.

FIG. 3 shows the transmission of this multilayer film. Curve (a) showsthe response at normal incidence for light polarized in the transmissiondirection, while curve (b) shows the response at 60 degrees forp-polarized light polarized in the transmission direction.

EXAMPLE 2 (PEN:PMMA, 601, Mirror)

A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. PolyethyleneNaphthalate (PEN) with an Intrinsic Viscosity of 0.57 dl/g (60 weightpercent phenol/40 weight percent dichlorobenzene) was delivered byextruder A at a rate of 114 pounds per hour with 64 pounds per hourgoing to the feedblock and the rest going to skin layers describedbelow. PMMA (CP-82 from ICI of Americas) was delivered by extruder B ata rate of 61 pounds per hour with all of it going to the feedblock. PENwas on the skin layers of the feedblock. The feedblock method was usedto generate 151 layers using the feedblock such as those described inU.S. Pat. No. 3,801,429, after the feedblock two symmetric skin layerswere coextruded using extruder C metering about 30 pounds per hour ofthe same type of PEN delivered by extruder A. This extrudate passedthrough two multipliers producing an extrudate of about 601 layers. U.S.Pat. No. 3,565,985 describes similar coextrusion multipliers. Theextrudate passed through another device that coextruded skin layers at atotal rate of 50 pounds per hour of PEN from extruder A. The web waslength oriented to a draw ratio of about 3.2 with the web temperature atabout 280 degrees F. The film was subsequently preheated to about 310degrees F in about 38 seconds and drawn in the transverse direction to adraw ratio of about 4.5 at a rate of about 11 percent per second. Thefilm was then heat-set at 440 degrees F with no relaxation allowed. Thefinished film thickness was about 3 mil.

As seen in FIG. 4, curve (a), the bandwidth at normal incidence is about350 nm with an average in-band extinction of greater than 99 percent.The amount of optical absorption is difficult to measure because of itslow value, but is less than 1 percent. At an incidence angle of 50percent from the normal both s (curve (b)) and p-polarized (curve (c))light showed similar extinctions, and the bands were shifted to shorterwavelengths as expected. The red band-edge for s-polarized light is notshifted to the blue as much as for p-polarized light due to the expectedlarger bandwidth for s-polarized light, and due to the lower index seenby the p-polarized light in the PEN layers.

EXAMPLE 3 (PEN:PCTG, 449, Polarizer)

A coextruded film containing 481 layers was made by extruding the castweb in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. The feedblock method was used with a 61 layerfeedblock and three (2×) multipliers. Thick skin layers were addedbetween the final multiplier and the die. Polyethylene naphthalate (PEN)with an intrinsic viscosity of 0.47 dl/g (60 weight percent phenol/40weight percent dichlorobenzene) was delivered to the feedblock by oneextruder at a rate of 25.0 pounds per hour. Glycol modified polyethylenedimethyl cyclohexane terephthalate (PCTG 5445 from Eastman) wasdelivered by another extruder at a rate of 25.0 pounds per hour. Anotherstream of PEN from the above extruder was added as skin layers after themultipliers at a rate of 25.0 pounds per hour. The cast web was 0.007inches thick and 12 inches wide. The web was layer uniaxially orientedusing a laboratory stretching device that uses a pantograph to grip asection of film and stretch it in one direction at a uniform rate whileit is allowed to freely relax in the other direction. The sample of webloaded was about 5.40 cm wide (the unconstrained direction) and 7.45 cmlong between the grippers of the pantograph. The web was loaded into thestretcher at about 100 degrees C and heated to 135 degrees C for 45seconds. Stretching was then commenced at 20 percent/second (based onoriginal dimensions) until the sample was stretched to about 6:1 (basedon gripper to gripper measurements). Immediately after stretching, thesample was cooled by blowing room temperature air at it. In the center,the sample was found to relax by a factor of 2.0.

FIG. 5 shows the transmission of this multilayer film where curve ashows transmission of light polarized in the non-stretch direction atnormal incidence, curve b shows transmission of p-polarized lightpolarized in the non-stretched direction at 60 degree incidence, andcurve c shows the transmission of light polarized in the stretchdirection at normal incidence. Average transmission for curve a from400-700 nm is 89.7 percent, average transmission for curve b from400-700 nm is 96.9 percent, and average transmission for curve c from400-700 nm is 4.0 percent. Percent RMS color for curve a is 1.05percent, and percent RMS color for curve b is 1.44 percent.

EXAMPLE 4 (PEN:CoPEN, 601, Polarizer)

A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. A Polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.54 dl/g (60 weightpercent Phenol plus 40 weight percent dichlorobenzene) was delivered byon extruder at a rate of 75 pounds per hour and the coPEN was deliveredby another extruder at 65 pounds per hour. The coPEN was a copolymer of70 mole percent 2,6 naphthalene dicarboxylate methyl ester, 15 percentdimethyl isophthalate and 15 percent dimethyl terephthalate withethylene glycol. The feedblock method was used to generate 151 layers.The feedblock was designed to produce a stack of films having athickness gradient from top to bottom, with a thickness ratio of 1.22from the thinnest layers to the thickest layers. The PEN skin layerswere coextruded on the outside of the optical stack with a totalthickness of 8 percent of the coextruded layers. The optical stack wasmultiplied by two sequential multipliers. The nominal multiplicationratio of the multipliers were 1.2 and 1.27, respectively. The film wassubsequently preheated to 310 degree F in about 40 seconds and drawn inthe transverse direction to a draw ratio of about 5.0 at a rate of 6percent per second. The finished film thickness was about 2 mils.

FIG. 6 shows the transmission for this multilayer film. Curve a showstransmission of light polarized in the non-stretch direction at normalincidence, curve b shows transmission of p-polarized light at 60 degreeincidence, and curve c shows transmission of light polarized in thestretch direction at normal incidence. Note the very high transmissionof p-polarized light in the non-stretch direction at both normal and 60degree incidence (80-100 percent). Also note the very high reflectanceof light polarized in the stretched direction in the visible range(400-700 nm) shown by curve c. Reflectance is nearly 100 percent between500 and 650 nm.

EXAMPLE 5 (PEN:sPS, 481, Polarizer)

A 481 layer multilayer film was made from a polyethylene naphthalate(PEN) with an intrinsic viscosity of 0.56 dl/g measured in 60 weightpercent phenol and 40 weight percent dichlorobenzene purchased fromEastman Chemicals and a syndiotactic polystyrene (sPS) homopolymer(weight average molecular weight=200,000 Daltons, sampled from DowCorporation). The PEN was on the outer layers and was extruded at 26pounds per hour and the sPS at 23 pounds per hour. The feedblock usedproduced 61 layers with each of the 61 being approximately the samethickness. After the feedblock three (2×) multipliers were used. Equalthickness skin layers containing the same PEN fed to the feedblock wereadded after the final multiplier at a total rate of 22 pounds per hour.The web was extruded through a 12 inch wide die to a thickness of about0.011 inches (0.276 mm). The extrusion temperature was 290 degrees C.

This web was stored at ambient conditions for nine days and thenuniaxially oriented on a tenter. The film was preheated to about 320degrees F (160 degrees C) in about 25 seconds and drawn in thetransverse direction to a draw ratio of about 6:1 at a rate of about 28percent per second. No relaxation was allowed in the stretcheddirection. The finished film thickness was about 0.0018 inches (0.046mm).

FIG. 7 shows the optical performance of this PEN:sPS reflectivepolarizer containing 481 layers. Curve a shows transmission of lightpolarized in the non-stretch direction at normal incidence, curve bshows transmission of p-polarized light at 60 degree incidence, andcurve c shows transmission of light polarized in the stretch directionat normal incidence. Note the very high transmission of p-polarizedlight at both normal and 60 degree incidence. Average transmission forcurve a over 400-700 nm is 86.2 percent, the average transmission forcurve b over 400-700 nm is 79.7 percent. Also note the very highreflectance of light polarized in the stretched direction in the visiblerange (400-700 nm) shown by curve c. The film has an averagetransmission of 1.6 percent for curve c between 400 and 700 nm. Thepercent RMS color for curve a is 3.2 percent, while the percent RMScolor for curve b is 18.2 percent.

EXAMPLE 6 (PEN:CoPEN, 603, Polarizer)

A reflecting polarizer comprising 603 layers was made on a sequentialflat-film making line via a coextrusion process. A polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.47 dl/g (in 60 weightpercent phenol plus 40 weight percent dichlorobenzene) was delivered byan extruder at a rate of 83 pounds (38 kg) per hour and the CoPEN wasdelivered by another extruder at 75 pounds (34 kg) per hour. The CoPENwas a copolymer of 70 mole percent, 2,6 naphthalene dicarboxylate methylester, 15 mole percent dimethyl terephthalate, and 15 mole percentdimethyl isophthalate with ethylene glycol. The feedblock method wasused to generate 151 layers. The feedblock was designed to produce astack of films having a thickness gradient from top to bottom, with athickness ratio of 1.22 from the thinnest layers to the thickest layers.This optical stack was multiplied by two sequential multipliers. Thenominal multiplication ratio of the multipliers was 1.2 and 1.4,respectively. Between the final multiplier and the die, skin layers wereadded composed of the same CoPEN described above, delivered by a thirdextruder at a total rate of 106 pounds (48 kg) per hour. The film wassubsequently preheated to 300 degrees F (150 degrees C) in about 30seconds and drawn in the transverse direction to a draw ratio ofapproximately 6 at an initial rate of about 20 percent per second. Thefinished film thickness was approximately 0.0035 inch (0.089 mm).

FIG. 8 shows the optical performance of the polarizer of Example 6.Curve a shows transmission of light polarized in the non-stretchdirection at normal incidence, curve b shows transmission of p-polarizedlight in the nonstretch direction at 50 degree angle of incidence, andcurve c shows transmission of light polarized in the stretch directionat normal incidence. Note the very high transmission of light polarizedin the non-stretch direction. Average transmission for curve a over400-700 nm is 87 percent. Also note the very high reflectance of lightpolarized in the stretched direction in the visible range (400-700 nm)shown by curve b. The film has an average transmission of 2.5 percentfor curve b between 400 and 700 nm. In addition, the percent RMS colorof this polarizer is very low. The percent RMS color for curve b is 5percent.

While the multilayer optical stacks, as described above, can providesignificant and desirable optical properties, other properties, whichmay be mechanical, optical, or chemical, are difficult to provide in theoptical stack itself without degrading the performance of the opticalstack. Such properties may be provided by including one or more layerswith the optical stack that provide these properties while notcontributing to the primary optical function of the optical stackitself. Since these layers are typically provided on the major surfacesof the optical stack, they are often known as “skin layers.”

A skin layer may be coextruded on one or both major surfaces of themultilayer stack during its manufacture to protect the multilayer stackfrom the high shear along the feedblock and die walls, and often anouter layer with the desired chemical or physical properties can beobtained by mixing an additive, such as, for example, a UV stabilizer,into the polymer melt that makes up the skin layer, and coextruding theskin layer with altered properties onto one or both sides of themultilayer optical stack during manufacture. Alternately, additionallayers may be coextruded on the outside of the skin layers duringmanufacture of the multilayer film; they may be coated onto themultilayer film in a separate coating operation; or they may belaminated to the multilayer film as a separate film, foil, or rigid orsemi-rigid reinforcing substrate such as polyester (PET), acrylic(PMMA), polycarbonate, metal, or glass. Adhesives useful for laminatingthe multilayer polymer film to another surface include both opticallyclear and diffuse adhesives and include both pressure sensitive andnon-pressure sensitive adhesives. Pressure sensitive adhesives arenormally tacky at room temperature and can be adhered to a surface byapplication of, at most, light finger pressure, while non- pressuresensitive adhesives include solvent, heat, or radiation activatedadhesive systems. Examples of adhesives useful in the present inventioninclude those based on general compositions of polyacrylate; polyvinylether; diene-containing rubber such as natural rubber, polyisoprene, andpolyisobutylene; polychloroprene; butyl rubber; butadiene-acrylonitrilepolymer; thermoplastic elastomer;. block copolymers such asstyrene-isoprene and styrene-isoprene-styrene block copolymers,ethylene-propylene-diene polymers, and styrene-butadiene polymer;poly-alpha-olefin; amorphous polyolefin; silicone; ethylene-containingcopolymer such as ethylene vinyl acetate, ethylacrylate, and ethylmethacrylate; polyurethane; polyamide; epoxy; polyvinylpyrrolidone andvinylpyrrolidone copolymers; polyesters; and mixtures of the above.Additionally, the adhesives can contain Ad additives such as tackifiers,plasticizers, fillers, antioxidants, stabilizers, pigments, diffusingparticles, curatives, biocides, and solvents. Preferred adhesives usefulin the present invention include VITEL 3300, a hot melt adhesiveavailable from Shell Chemical Co. (Akron, Ohio), or an acrylic pressuresensitive adhesive such as a 90/10 IOA/AA acrylic adhesive fromMinnesota Mining and Manufacturing Company, St. Paul, Minnesota. When alaminating adhesive is used to adhere the multilayer film to anothersurface, the adhesive composition and thickness are preferably selectedso as not to interfere with the optical properties of the multilayerstack. For example, when laminating additional layers to a multilayerpolymer polarizer or mirror wherein a high degree of transmission isdesired, the laminating adhesive should be optically clear in thewavelength region that the polarizer or mirror is designed to betransparent.

FIGS. 10 and 11 illustrate multilayer stacks having respectively one andtwo additional layers, respectively. FIGS. 10 and 11 will be used belowto describe a variety of additional layers that could be applied.

One area in which a skin layer having differing mechanical properties isdesirable relates particularly to uniaxially oriented multilayer opticalstacks, such as reflective polarizers. Such stacks often tend to show alow tear resistance in the principal draw direction. This can lead toreduced yields during the manufacturing process or to subsequentbreakage of the film during handling. In order to resist this, tearresistant layers may be adhered to the outer major surfaces of theoptical stack. These tough layers may be of any appropriate material andcould even be the same as one of the materials used in the opticalstack. Factors to be considered in selecting a material for a tearresistant layer include percent elongation to break, Young's modulus,tear strength, adhesion to interior layers, percent transmittance andabsorbance in an electromagnetic bandwidth of interest, optical clarityor haze, refractive indices as a function of frequency, texture androughness, melt thermal stability, molecular weight distribution, meltrheology and coextrudability, miscibility and rate of inter-diffusionbetween materials in the tough and optical layers, viscoelasticresponse, relaxation and crystallization behavior under draw conditions,thermal stability at use temperatures, weatherability, ability to adhereto coatings and permeability to various gases and solvents. Of course,as previously stated, it is important that the material chosen not haveoptical properties deleterious to those of the optical stack. They maybe applied during the manufacturing process or later coated onto orlaminated to the optical stack. Adhering these layers to the opticalstack during the manufacturing process, such as by a coextrusionprocess, provides the advantage that the optical stack is protectedduring the manufacturing process.

Using FIG. 10 to illustrate this aspect of the invention, a multilayeroptical stack having tear resistant layers 400 is shown. Film 400includes an optical stack 410. Optical stack 410 includes alternatinglayers 412 and 414 of two polymers having differing optical properties.Attached to the major surfaces of optical stack 410 are tear resistantlayers 416 and 418. It should be noted that, although layers 416 and 418are shown in FIG. 10 as thicker than layers 412 and 414, FIG. 10 is notto scale for a generally preferred embodiment. In general it isdesirable that each of layers 416 and 418 have a thickness greater than5 percent of the thickness of the optical stack. It is preferred thateach of layers 416 and 418 have a thickness in the range of 5 percent to60 percent of the thickness of the optical stack to provide tearresistance without unnecessarily increasing the amount of material used.Thus, if the optical stack has 600 layers, in such a preferredembodiment the thickness of each of tear resistant layers 416 and 418would be equal to the thickness of 30 to 360 of the layers of the stack.In a more preferred embodiment each of the tear resistant layers 416 and418 would have a thickness in the range of 30 percent to 50 percent ofthat of the optical stack.

In a particularly desirable embodiment, tear resistant outer layers maybe of one of the same materials used in alternating layers 412 and 414.In particular, it has been discovered that in a reflective polarizercomprising alternating layers of PEN and coPEN, tear resistant outerlayers of coPEN may be coextruded during the manufacturing process.

EXAMPLE 7

A multilayered composite of alternating PEN and coPEN layers to form areflective polarizer was coextruded with thick skin layers of coPEN toform a tear resistant reflective polarizer. A coextruded film containing603 layers was made on a sequential flat-film extruder. A polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.47 dl/g (in 60 weightpercent phenol plus 40 weight percent dichlorobenzene) was delivered byan extruder at a rate of 86 pounds per hour and the coPEN was deliveredby another extruder at 78 pounds per hour. The coPEN was a copolymer of70 mole percent, 2,6 naphthalene dicarboxylate methyl ester and 30percent dimethyl terephthalate with ethylene glycol. The feedblockextruded 151 layers. The feedblock was designed to produce a stack offilms having a thickness gradient from top to bottom, with a thicknessratio of 1.22 from the thinnest layers to the thickest layers. Thisoptical stack was multiplied by two sequential multipliers. The nominalmultiplication ratio of the multipliers was 1.2 and 1.27, respectively.Between the final multipliers and the die, composed of coPEN asdescribed above, layers were added. These layers were charged anddelivered by a third extruder at a total rate of 187 pounds per hour.The film with the additional coPEN outer layers was preheated to 320degrees F in about 40 seconds and drawn in the transverse direction to adraw ratio of approximately 6 at an initial rate of about 20 percent persecond. The finished film had a thickness of approximately 100 μmincluding an inner multilayered optical stack of about 50 μm thicknessand two exterior outer layers (one on each side of the film) of about 25μm thickness, each. Tear resistance improved over the case without skinsallowing the creation of wound rolls of tough reflective polarizer.Specifically, tear resistance was measured on films made according tothis example and on film made under similar conditions but without coPENskin layers using a trouser tear test along the principal drawdirection, according to ASTM D-1938. Average film thicknesses were 100μm and 48 μm, respectively. The average tear force values were 60.2 and2.9 grams force, with standard deviations of 4.44 and 0.57 grams force,respectively. Analysis of the coPEN skin layers showed low orientationwith indices of refraction of 1.63, 1.62, and 1.61 at 633 nm. Goodinterlayer adhesion was demonstrated by the difficulty of cleanlyseparating the construction. For further comparison a 48 μm opticalstack having 3.8 μm outer layers of PEN was tested and found to have anaverage tear force of 2.8 grams with a standard deviation of 1.07.

The appearance and/or performance of a film may be altered by includinga skin layer having a dye or pigment that absorbs in one or moreselected regions of the spectrum. This can include portions or all ofthe visible spectrum as well as ultraviolet and infrared. Of course, ifall of the visible spectrum is absorbed, the layer will be opaque. Thesecan be selected in order to change the apparent color of lighttransmitted or reflected by the film. They can also be used tocompliment the properties of the film, particularly where the filmtransmits some frequencies while reflecting others. The use of an UVabsorptive material in a cover layer is particularly desirable becauseit may be used to protect the inner layers that may be unstable whenexposed to UV radiation. Thus, FIG. 9 illustrates such a film with layer316 representing a layer containing an electromagnetic absorbingmaterial.

Similar to the electromagnetic absorbing materials described above, afluorescent material could be incorporated in layer 316 of FIG. 9 or oneor both of layers 416 and 418 of FIG. 9. Fluorescent materials absorbelectromagnetic energy in the ultraviolet region of the spectrum andreemit in the visible. Desirable fluorescent materials include hinderedamine light stabilizers (HALS) and are described in more detail in U.S.patent application Ser. No. 08/345,608, filed Nov. 28, 1994, thedisclosure of which is incorporated herein by reference.

Pressure sensitive adhesives form another desirable class of materialsthat may be applied to a multilayer stack as layer 316 of FIG. 9 or oneof layers 416 or 418 of FIG. 10. Generally pressure sensitive adhesivesmay be applied when the optical stack is intended for later laminationto another material, such as a glass or metal substrate.

Another material that could be incorporated in a skin layer such aslayer 316 or one of layers 416 or 418 would be a slip agent. A slipagent will make the film easier to handle during the manufacturingprocess. Typically a slip agent would be used with a mirror film ratherthan a film intended to transmit a portion of the light striking it. Theside including the slip agent would typically be the side intended to belaminated to a supporting substrate in order to prevent the slip agentfrom increasing haze associated with the reflection.

Another type of additional layer that could be used is a protectivelayer. Such a layer could be abrasion resistant or resistant toweathering and/or chemical action. Such coatings would be particularlyuseful in situations where the multilayer film is to be exposed to aharsh or corrosive environment. Examples of abrasion-resistant or hardcoatings include acrylic hardcoats such as Acryloid A-11 and ParaloidK-120N, available from Rohm & Haas; urethane acrylates, such asdescribed in U.S. Pat. No. 4,249,011 and those available from SartomerCorp.; and urethane hardcoats such as those obtained from reacting analiphatic polyisocyanate such as Desmodur N-3300, available from Miles,Inc. with a polyester such as Tone Polyol 0305, available from UnionCarbide. Such layers could also provide protection against transmissionof gases such as oxygen or carbon dioxide or water vapor through thefilm. Again this could be a single layer as shown in FIG. 9 or layers onboth sides as shown in FIG. 10.

Other layers that could be added include layers containing holographicimages, holographic diffusers, or other diffusing layers. Such layerscould be in a hard polymer or in an adhesive

FIG. 11 shows alternative multilayer film 500 having alternating layers512 and 514 with protective layers 516, 518, and 520. Thus, multipleadditional layers could be provided adjacent a single major surface ofthe multilayer optical stack. An example of a use for a structure of thetype shown in FIG. 11 would be one in which protective layers 516 and518 were tear resistant structures, as described above, and layer 520was abrasion resistant..

The foregoing has been examples of various coatings that could beapplied to the exterior of a multilayer stack to alter its properties.In general, any additional layers could be added that would havedifferent mechanical, chemical, or optical properties than those of thelayers of the stack itself.

What is claimed is:
 1. A multilayer film including an optical stackcomprising layers of a semi-crystalline polymer having an averagethickness of not more than 0.5 microns and layers of a second polymerhaving an average thickness of not more than 0.5 microns wherein saidoptical stack has been stretched in at least one direction to at leasttwice that direction's unstretched dimension, said optical stack havingfirst and second major surfaces, each of said layers having indices ofrefraction n_(x) and n_(y) in a plane of said layer and n_(z) normal toa plane of said layer all of said indices of refraction being selectedto provide desired optical properties, said film further comprising afirst additional layer adhered to said first major surface, saidadditional layer being of a material selected for its mechanicalproperties, said mechanical properties differing from mechanicalproperties of said layers of said optical stack.
 2. The multilayer filmof claim 1 further comprising a second additional layer adhered to saidsecond major surface.
 3. The multilayer film of claim 1 further having asecond additional layer adhered to said first additional layer, saidsecond additional layer having mechanical properties differing fromthose of said layers of said optical stack and those of said firstadditional layer.
 4. The multilayer film of claim 3 wherein said firstadditional layer is a tear resistant layer and said second additionallayer is an abrasion resistant layer.
 5. The multilayer film of claim 1wherein said second additional layer is an abrasion resistant layer. 6.The multilayer film of claim 1 wherein said first additional layerincludes a slip agent.
 7. A multilayer optical film having layers offirst and second polymers, said first and second polymers differing incomposition, each of said layers having a thickness of no more than 0.5microns, said layers of said first polymer having indices of refractionof n_(1x) and n_(1y) in planes of said layers of said first polymer andn_(1z) normal to said planes of said layers of said first polymer andsaid layers of said second polymer having indices of refraction ofn_(2x), and n_(2y) in planes of said layers of said second polymer andn_(2z) normal to said planes of said layers of said second polymer layerall of said indices of refraction being selected to provide desiredoptical properties, said optical stack having first and second majorsurfaces, said first major surface having adhered thereto a first tearresistant layer.
 8. The multilayer optical film of claim 7 wherein saidsecond major surface has adhered thereto a second tear resistant layer.9. The multilayer optical film of claim 8 wherein each of said tearresistant layers has a thickness greater than 5 percent of the thicknessof said optical stack.
 10. The multilayer optical film of claim 9wherein each of said tear resistant layers has a thickness in the rangeof 5 percent to 60 percent of the thickness of said optical stack. 11.The multilayer optical film of claim 10 wherein each of said tearresistant layers has a thickness in the range of 30 percent to 50percent of the thickness of said optical stack.
 12. The multilayeroptical film of claim 9 wherein said tear resistant layers have acomposition that is the same as the composition of said second polymers.13. The multilayer optical film of claim 12 wherein said first polymeris polyethylene naphthalate and said second polymer is a copolyestercomprising naphthalate and terephthalate units.
 14. The multilayeroptical film of claim 7 wherein said first polymer has a positive stresscoefficient.
 15. A multilayer film including an optical stack comprisinglayers of a semi-crystalline polymer having an average thickness of notmore than 0.5 microns and layers of a second polymer having an averagethickness of not more than 0.5 microns wherein said optical stack hasbeen stretched in at least one direction to at least twice thatdirection's unstretched dimension, said optical stack having first andsecond major surfaces, each of said layers having indices of refractionn_(x) and n_(y) in a plane of said layer and n_(z) normal to a plane ofsaid layer all of said indices of refraction being selected to providedesired optical properties, said film further comprising a firstadditional layer adhered to said first major surface, said additionallayer being of a material selected for its chemical properties, saidchemical properties differing from chemical properties of said layers ofsaid optical stack.
 16. The multilayer film of claim 15 furthercomprising a second additional layer adhered.
 17. A multilayer filmincluding an optical stack comprising layers of a semi-crystallinepolymer having an average thickness of not more than 0.5 microns andlayers of a second polymer having an average thickness of not more than0.5 microns wherein said optical stack has been stretched in at leastone direction to at least twice that direction's unstretched dimension,said optical stack having first and second major surfaces, each of saidlayers having indices of refraction n_(x) and n_(y) in a plane of saidlayer and n_(z) normal to a plane of said layer all of said indices ofrefraction being selected to provide desired optical properties, saidfilm further comprising a first additional layer adhered to said firstmajor surface, said additional layer being of a material selected forits optical properties, said optical properties differing from opticalproperties of said layers of said optical stack.
 18. The multilayer filmof claim 17 further comprising a second additional layer adhered. 19.The multilayer film of claim 17 wherein said first additional layerincludes an electromagnetic absorbing material.
 20. The multilayer filmof claim 19 wherein said electromagnetic absorbing material absorbsultraviolet radiation.
 21. The multilayer film of claim 17 wherein saidfirst additional layer includes a fluorescent material.
 22. Themultilayer film of claim 17 wherein said first additional layer includesa holographic image.
 23. The multilayer film of claim 17 wherein saidfirst additional layer includes a holographic diffuser.
 24. A multilayerfilm including an optical stack comprising layers of a semi-crystallinepolymer having an average thickness of not more than 0.5 microns andlayers of a second polymer having an average thickness of not more than0.5 microns wherein said optical stack has been stretched in at leastone direction to at least twice that direction's unstretched dimension,said optical stack having first and second major surfaces, each of saidlayers having indices of refraction n_(x) and n_(y) in a plane of saidlayer and n_(z) normal to a plane of said layer all of said indices ofrefraction being selected to provide desired optical properties, saidfilm further comprising a first additional layer adhered to said firstmajor surface, said additional layer being of a pressure sensitiveadhesive.
 25. A multilayer film including an optical stack comprisinglayers of a semi-crystalline polymer having an average thickness of notmore than 0.5 microns and layers of a second polymer having an averagethickness of not more than 0.5 microns wherein said optical stack hasbeen stretched in at least one direction to at least twice thatdirection's unstretched dimension, said optical stack having first andsecond major surfaces, each of said layers having indices of refractionn and ny in a plane of said layer and n normal to a plane of said layerall of said indices of refraction being selected to provide desiredoptical properties, said film further comprising a first additionallayer adhered to said first major surface, said additional layer beingof a material selected for its mechanical properties, said mechanicalproperties differing from mechanical properties of said layers of saidoptical stack wherein said additional layer is a tear resistant layer inthe range of 5 percent to 60 percent of the thickness of said opticalstack has a composition that is the same as the composition of saidsecond polymer.
 26. The multilayer optical film of claim 25 wherein saidfirst polymer is polyethylene naphthalate and said second polymer is acopolyester comprising naphthalate and terephthalate units.
 27. Amultilayer film including an optical stack comprising layers of asemi-crystalline polymer having an average thickness of not more than0.5 microns and layers of a second polymer having an average thicknessof not more than 0.5 microns wherein said optical stack has beenstretched in at least one direction to at least twice that direction'sunstretched dimension, said optical stack having first and second majorsurfaces, each of said layers having indices of refraction n_(x) andn_(y) in a plane of said layer and n_(z) normal to a plane of said layerall of said indices of refraction being selected to provide desiredoptical properties, said film further comprising a first additionallayer adhered to said first major surface, said additional layer beingof a material selected for its mechanical properties, said mechanicalproperties differing from mechanical properties of said layers of saidoptical stack wherein said additional layer is glass.